1. Field of the Invention
The present invention relates to a system for shifting buoyancy to leeward in a nautical vessel to increase righting moment and reduce heel.
2. Description of the Prior Art
Nautical vessels adapted for propulsion through the water all respond to transverse components of the force of the wind by heeling. In the case of a sailing vessel which is propelled forward by the force of wind the effects of heeling have a considerable impact on the speed with which the vessel can travel under any given wind conditions, as well as the extent to which the vessel can point into the wind.
Conventional sailing vessels may have either a monohull or a multiple hull configuration. A monohull sailing vessel has a single, elongated hull which is narrowest at the bow, broadens amidships, and narrows somewhat at the stern. The surface of the hull is streamlined so as to minimize water resistance as the hull travels through the water.
Monohull sailing vessels invariably employ some type of keel. The keel of most modern sailing vessels is formed as a thin, relatively narrow slab-like structure, which is streamlined at its fore and aft edges, and which extends downwardly from the bottom of the hull at some location amidship. The keel is formed of some heavy material, such as steel or lead.
The keel performs several important functions. Because a keel is relatively broad in a fore and aft direction, it presents a very large surface area perpendicular to the direction of forward travel of the vessel. The keel thereby offers considerable resistance to the transverse components of the wind and wave forces acting normal to the desired direction of travel of the vessel. The relatively large resistance to transverse or lateral motion by the keel limits the extent to which the vessel is pushed sideways in the water perpendicular to its intended direction of travel.
The keel also performs an extremely important function by providing ballast which creates a righting moment. This moment varies depending upon the extent to which a vessel is tilted by the transverse or lateral component of wind acting against the sails of the vessel. The force component of the wind in the sails acting perpendicular to the fore and aft alignment of the hull cause the vessel to tilt or cant to one side. This is termed heeling. Since the water resists sideways movement of the hull, the top of the vessel will tilt or heel at an incline away from the direction of the wind. The force of the wind acting normal to the desired direction of travel of the vessel thereby creates a moment which, if unopposed, could force the masts of the vessel into the water and capsize the vessel. However, when the top of the mast of a vessel is pushed to the direction toward which the wind is moving, which is the leeward direction, the rotational moment exerted on the hull causes the keel to rise upward in the water on the windward side of the hull.
Due to its very large weight, the keel of the vessel counters the heeling force of the wind by exerting an opposing moment on the vessel in a direction opposite to that exerted by the transverse component of the wind. The more powerful the force of the wind, the greater will be the heeling force on the vessel and the more the keel will rise closer to the horizontal. As the keel rises its moment arm or lever arm for the ballast it contains increases, thereby increasing the righting moment which the keel exerts on the vessel. Conversely, as the vessel heels the extent to which the transverse component of the wind acts normal to the sails is reduced. The vessel therefore reaches some equilibrium angle of heel at which the transverse moment exerted on the upper portion of the vessel by the wind is countered by an opposing transverse moment exerted on the lower portion of the vessel by the keel.
A sailing vessel operates most efficiently when it is heeled to somewhat. However, if the vessel heels to severely, it will spill the wind, thus reducing the driving force of the sails and reducing the speed with which the vessel is propelled through the water. The optimum extent of heeling varies from one vessel to another, but for monohull sailing vessels the optimum angel of heel is typically between about 10 degrees and 25 degrees from the vertical. Most sailing vessels do not operate efficiently in traveling at an angle to the wind if they heel less than about 8 degrees or greater than about 30 degrees.
While sailors can reduce heeling by reducing the sail area carried aloft, speed is inevitably sacrificed since the wind strength invariably lightens intermittently and momentarily. Therefore, when the sail area carried aloft is optimum for an average wind speed, momentary lulls in the wind detract from the driving force of the sails. Sailors therefore tend to carry enough sail area aloft to take advantage of momentary gusts of wind. With conventional sailing vessel designs there is therefore a continuous, recurring problem of excessive heeling, especially in gusty, heavy wind conditions.